LONDON — Police investigating a wave of transit system bomb attacks yesterday arrested seven more persons and separately uncovered a large cache of faked identity documents, spurring fears that additional terror cells are waiting to strike.

The security services are particularly worried that fresh attacks could be scheduled for Thursday, exactly four weeks after the first set of bombings and two weeks after an attempt to duplicate those bombings.

Yesterday’s arrests were made in Brighton, an upscale coastal town in southern England, where police arrested six men and a woman in two operations. Their connection to the bomb plot was not disclosed.

Unlike the dramatic arrests of three bombing suspects Friday, yesterday’s operations took place without an armed assault team. The arrests bring to 19 the number of persons held in connection with the botched attacks, including the four suspected of being the would-be bombers.

Police yesterday were interrogating those four and a fifth suspect who is thought to have abandoned an explosive charge in a park.

Much interest has focused on a cell-phone call made to Saudi Arabia by Ethiopian-born refugee Osman Hussain, 27, accused of being one of the four would-be bombers, shortly before he was arrested along with his brother Friday in Rome.

Senior police officers said they were piecing together a possible pyramid network behind both sets of bombers, including the possibility that one or more masterminds is still at large in Britain.

“We fear these handlers who have not been caught could be controlling other suicide gangs ready to strike,” said one security source, who spoke on the condition of anonymity.

The source offered no substantive evidence of additional cells, but did reveal that a bag found near Heathrow airport, Europe’s largest, was packed with enough fake passports and bogus identity documents to have supplied several terrorist cells.

The travel bag was stuffed with visas, forged documents supposedly from the British Home Office, bank cards and work permits. It was handed over to police after a taxi driver found them and passed them to investigative journalist Mazher Mahmood.

The British internal security service also faced criticism after it was revealed that Mr. Hussain had managed to escape the country aboard the Eurostar train through the Channel Tunnel to France. He traveled using his regular British passport five days after the failed July 21 bombings.

In Italy, police arrested Mr. Hussain’s brother, Remzi Isaac, who owns an African souvenir shop near Rome’s main railway station, on charges of keeping false documents. Italian police also found a variety of air tickets for various European destinations together with a British passport in the name of a second brother of Mr. Hussain’s.

British authorities were expected to ask today for Mr. Hussain’s extradition under an expedited European arrest warrant.

British interrogators said it has been “slow going” to extract information from a suspected bomber who was arrested in Birmingham early last week.

The suspect has been permitted to have a lawyer present and is entitled to numerous breaks — including five a day for Muslim prayers. He has also complained that he is disoriented from the effects of a stun gun that was fired at him during his arrest.

Italian newspapers, however, have been full of details from the questioning of Mr. Hussain in Rome, where interrogations are conducted by investigating magistrates.

The suspect has described instructions from a “boss” who told the men to wear rucksacks that would lead to “demonstrative attacks” in which “we didn’t want to kill, we just wanted to scare people.”

However, he had also been reported to have said: “We were supposed to blow ourselves up.”

Mr. Hussain said the men were shown videos, accompanied with dramatic Arabic music, of grief-stricken Iraqi widows and children alongside shots of Iraqi women and children who supposedly were killed by U.S. and British troops.

He reportedly told interrogators that after watching the videos, “there was a feeling of hatred and a conviction that it was necessary to give a signal — to do something.”